This invention relates to the production of glass that has low infrared energy transmittance and relatively high visible light transmittance. Such a glass is useful in glazing vision openings for the sake of reducing air conditioning requirements without unduly impairing vision, and is particularly applicable for vehicle windows.
The passage of infrared radiation through glass windows is a major cause of heat buildup in enclosed spaces such as automobiles. The accumulation of heat is, in turn, undesirable because of the burden placed on the air conditioning system, or because of the discomfort caused in occupants with or without air conditioning. The conventional approach has been to use "tinted" glass in such applications, which is usually darker green in color than ordinary "clear" glass due to a larger amount of iron included in the glass during melting. The iron renders the glass more absorptive of radiation in the infrared range of wavelengths (greater than 700 nanometers) and also reduces the visible light (luminous) transmittance. Conventional soda-lime-silica flat glass products tinted with iron typically transmit about 25 to 30 percent of the infrared radiation incident on a 5 millimeter thick sheet, and recently some products adapted to reduce infrared transmittance transmit less, approaching 15 percent transmittance levels. It would be desirable to reduce infrared transmittance levels even further, below the 15 percent level, without unduly decreasing luminous transmittance.
It is known in the art that infrared transmittance can be further reduced by including larger amounts of iron in the glass, but luminous transmittance is also reduced below levels considered desirable for adequate vision or for aesthetic purposes. It would be preferred to maintain luminous transmittance about 65 percent, preferably at least 70 percent. It is known that iron in the ferrous state (Fe.sup.+2) is largely responsible for absorption of infrared energy in glass (W. A. Weyl, "Coloured Glass," page 91). Therefore, attaining lower infrared transmittance without substantially reducing luminous transmittance would theoretically be possible by maintaining reducing conditions during the glassmaking process so as to increase the amount of iron in the ferrous state for a given total iron concentration. Unfortunately, such an approach has significant drawbacks for commercial production of glass.
The automotive and architectural glass markets, to which infrared absorbing glass is directed, require mass production on a large scale, with the necessity of melting, refining, and forming the glass on a continuous basis. Most large scale production of glass is carried out in overhead fired, tank type, continuous melting furnaces. When the glass is a reduced condition so as to enhance the proportion of iron in the ferrous state, the glass becomes so absorptive that penetration of heat into the body of molten glass is rendered very difficult. The result is substantially reduced thermal efficiency, and at higher ferrous levels adequate melting and refining becomes impractical or impossible in a conventional furnace. A typical tinted glass with a ferrous to total iron ratio of about 25 percent (ferrous iron expressed as FeO and total iron expressed as Fe.sub.2 O.sub.3) strains the ability of a commercial furnace to produce adequately melted and refined glass. Ferrous to total iron ratios in excess of 35 percent would heretofore have been considered unfeasible for continuous commercial flat glass production.
Another drawback for producing reduced glass on a continuous commercial basis is the conventional presence of substantial amounts of sulfur in soda-lime-silica glass, especially flat glass. Sulfur, typically included in the batch materials as a sulfate and analyzed in the glass as SO.sub.3, is present as a melting and refining aid. Although much of the sulfur volatilizes during melting and refining, conventional commercially produced flat glass has a residual SO.sub.3 content greater than 0.1 percent by weight of the glass, usually about 0.2 percent. In a glass composition that includes iron and sulfur, providing reducing conditions is known to create amber coloration which substantially lowers the luminous transmittance of the glass. In "Colour Generation and Control in Glass" by C. R. Bamford (Elsevier, 1977), on page 106, it is stated that "A rich golden-brown or amber colour is produced by the combination of sulphur and iron oxide in a soda-lime-silica glass melted under strongly reducing conditions." It is further stated on page 107 that "Onset of the amber colouration occurs at a ferrous value of 50 percent . . . " Therefore, in commercial flat glass manufacturing operations, the reliance on sulfur as a melting and refining aid has limited the degree to which the ferrous concentration of the glass could be increased to lower the infrared transmittance without unacceptably reducing the luminous transmittance. It would be desirable to be able to produce flat glass commercially with a ferrous content greater than 50 percent of the total iron content so as to minimize the total amount of iron needed to yield the desired infrared absorption.
Much of the published information on infrared absorbing glass is based on small scale, discontinuous, laboratory melts in which the commercial scale problems of achieving adequate melting and refining are usually not addressed. Small scale melts usually do not entail problems such as penetration of heat into a substantial depth of melt, limited residence time, homogenization of impurities from mineral batch materials or vessel erosion, and the presence of refining aids. This is because a batch-wise melting of a crucible or not of glass may be provided with indefinite melting times, may involve non-contaminating vessels of a material such as platinum, and may utilize purified grades of chemical compounds as raw materials. In the past, pot melts of glass having a desirable combination of infrared and luminous transmittance properties were produced in sufficient quantities to be cast, rolled, ground, and polished to produce flat glass plates that were marketed. Some of these melts had ferrous to total iron ratios between 40 percent and 50 percent. These pot melted glass compositions required long melting and refining times, were difficult to refine in spite of the user of sulfur refining aid, and were considered unsuitable for continuous flat glass production.
Japanese patent publication No. 60215546 (1985) has as its object a transparent, infrared absorbing glass wherein substantial amounts of barium oxide are included in the glass to shift the absorption peak toward the infrared wavelengths. However, barium oxide is a costly batch material, and it would be desirable to avoid the inconvenience of handling an additional batch constituent. Furthermore, it is taught that in glass in which sulfur is present as a refining aid, as would be the case with most commercially produced flat glass, substantial amount of zinc oxide should be included to prevent the formation of amber coloration when reducing conditions are imposed. But glass containing zinc oxide has been found to be incompatible with the float process, by which most flat glass is produced. This is due to the volatility of zinc oxide in the float forming chamber, which not only contaminates the interior of the chamber, but also leads to amber streaks in the glass where the zinc oxide content has been depleted.
Incompatibility with the float process also prevents the use of alternative refining aids such as antimony oxide or arsenic oxide instead of sulfur. Glass containing those constituents tend to discolor when brought into contact with molten tin in the float process. Fluorine and chlorine are also sometimes considered as alternatives to sulfur, but their volatility and associated environmental problems discourage their use.
U.S. Pat. No. 3,652,303 (Janakirama Rao) discloses the production of a reduced, heat absorbing glass by inclusion of tin oxide and chlorine in the glass. Providing tin as a substantial batch ingredient significantly increases the cost of the glass, and the volatility problems of chlorine are a drawback. It would be desirable if the combination of high visible light transmittance and low infrared transmittance could be attained with glass compositions not significantly different from stand, commercial, soda-lime-silica glass. It also appears that the Janakirama Rao glass compositions would not lend themselves to manufacture in a conventional continuous melting furnace.
Reducing the amount of transmitted ultraviolet radiation is also a desirable feature for the sake of reducing the fading of fabrics and other interior components. Japanese patent publication No. 61136936 (Asahi Glass) provides titanium dioxide to improve the ultraviolet blocking properties of glass and asserts that reduction in infrared transmittance is also achieved. However, the effect of titanium dioxide on infrared transmittance is less than desired as evidenced by the total solar energy transmittance of 51 percent reported in the Japanese patent document for five millimeter thick glass. Since infrared transmittance is the major component of total solar energy transmittance, the total solar energy transmittance of a satisfactory infrared absorbing glass would be less than 50 percent and preferably less than 40 percent. The primary object of the present invention is to provide low infrared transmittance, but additionally providing low ultraviolet transmittance would also be desirable.